This invention relates to an improvement in a meter monitor apparatus for monitoring the position of a mechanical indicator or pointer and in particular to such a device for monitoring the pressure and/or flow of a fluid between a high value, a low value or both and producing an output indicative of the pressure and/or flow condition.
Gas flowmeters, as well as other fluid flowmeters, are available to monitor the rate of flow. For example, continuous supervision has been provided by alarm devices to detect too low of flow of hydrogen, disassociated ammonia as well as natural carrier and other gases to furnaces. Similary, excessive flow in use of costly gases, such as argon and helium, may be detected. A particularly satisfactorily gas flowmeter and alarm or monitor unit is disclosed in U.S. Pat. No. 3,150,360 which issued Sept. 22, 1964 to Wallace L. Stenzel and is assigned to the present assignee herewith. As more fully disclosed therein, a photoelectric monitor or alarm unit includes an incandescent lamp unit and a photosensitive unit such as a photocell mounted to the opposite sides of a transparent tube coupled to the flow line. The tube is filled with a liquid and a pointer is mounted within the tube and coupled to the flow system for positioning in the liquid in accordance with the flow rate. A tube guard is telescoped over the tube and the alarm unit is clamped thereto with the lamp unit and the photocell aligned with oppositely located slots for transmission of the light beam. The pointer includes at least one opaque section operable to move through the light beam as the flow rate varies through a particular level. The lamp unit includes a special separate lens unit secured between the lamp and the tube guard slot for focusing the light beam onto the tube. The photocell includes an aperature aligned with the light beam for concentrating the beam onto a light sensitive area of the photocell unit. The lamp and the photocell units include outer housings which are mounted on a clamping support. A clamping bolt is threaded in the support with a clamping bracket on its inner end which moves into engagement with the guard to clamp the alarm device in position. The alarm device is located on the guard to respond to a particular flow rate indication. Although shown therein using a vacuum tube circuit, a light sensitive controlled rectifier has been used to control a solid state relay.
Although the prior art photoelectric systems have provided a reliable monitoring means, the systems are subject to malfunction, such as in the event the float oil becomes contaminated. Further, the systems tend to produce an unstable response and output in the presence of slow changes in the flow rate at the control limit. Thus, a chattering type response is at times encountered under such conditions of slow changes in the fluid state. Certain applications are of course more critical than others. For example, sintering furnaces using hydrogen gas atmosphere must be fully monitored for loss of hydrogen gas, because any such loss may allow entry of air, creating a possible explosive condition. Other applications which use rare and costly gases are monitored for most efficient usage because of the cost factors. In other systems, a similar requirement of pressure detection may be required. A float system may of course provide accurate detection of pressure, but a suitable transducer must again be provided.
There is therefore a need for a sensitive control for reliable detection of change in gas flow conditions through a selected limit.